Raft Footing Reinforcement in BBS

Raft footing, also known as mat footing, is a large continuous slab designed to support multiple columns and walls. It is used where the soil bearing capacity is low, or loads from the structure are distributed over a large area. Preparing the Bar Bending Schedule (BBS) for raft footings ensures proper estimation of steel requirements and aids in efficient construction planning.

Components of Raft Footing Reinforcement

  • Main Reinforcement: Bars placed in the longer direction of the raft to resist bending moments.
  • Distribution Reinforcement: Bars placed perpendicular to the main reinforcement to distribute loads evenly.
  • Shear Reinforcement: Additional reinforcement to resist shear forces near column locations.

Example of Raft Footing BBS

Given:

    Raft size: 8m x 6m x 0.6m
    Cover: 75mm
    Main bar diameter: 20mm
    Distribution bar diameter: 16mm
    Spacing of bars: 200mm c/c

Step 1: Calculate Number of Bars

    Effective length = Total length - 2 x Cover
                    = 8000mm - 2 x 75mm = 7850mm
    
    Effective width = Total width - 2 x Cover
                   = 6000mm - 2 x 75mm = 5850mm
    
    Number of main bars = (Effective width / Spacing) + 1
                       = (5850 / 200) + 1 = 30 bars
    
    Number of distribution bars = (Effective length / Spacing) + 1
                                = (7850 / 200) + 1 = 40 bars

Step 2: Calculate Bar Length

    Length of each main bar = Effective length + 2 x Cover
                           = 7850 + 2 x 75 = 8000mm or 8m
    
    Length of each distribution bar = Effective width + 2 x Cover
                                   = 5850 + 2 x 75 = 6000mm or 6m

Step 3: Total Steel Quantity

    Total length of main bars = Number of main bars x Length of each main bar
                             = 30 x 8 = 240m
    
    Total length of distribution bars = Number of distribution bars x Length of each distribution bar
                                      = 40 x 6 = 240m

Best Practices

  • Ensure adequate cover to protect reinforcement from corrosion and environmental factors.
  • Follow design specifications for bar placement and spacing.
  • Verify overlap length for bars if the length is insufficient.
  • Provide additional reinforcement near column locations for shear resistance.

By preparing an accurate BBS for raft footings, structural engineers can ensure material optimization, cost efficiency, and structural stability.




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